Fibrillar adhesives on the feet of geckos and spiders and other animals have been of great interest because they can repeatedly attach to wide range of surfaces with a controllable adhesion strength in various environments including vacuum, and leave no residue. Furthermore, fibrillar adhesives are self-cleaning which allows for long lifetime and repeated use without significant performance degradation [W. Hansen and K. Autumn. Evidence for self-cleaning in gecko setae. Proceedings of the National Academy of Sciences, 102:385-389, 2005.]. These foot-hairs conform to the surface roughness to increase the real contact area, resulting in high adhesion by surface forces [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full. Adhesive force of a single gecko foot-hair. Nature, 405:681-685, 2000.]. This adhesion, called dry adhesion, is argued to arise from molecular surface forces such as van der Waals forces [K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full. Evidence for van der waals adhesion in gecko setae. Proceedings of the National Academy of Sciences, 99:12252-56, September 2002.], [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full. Adhesive force of a single gecko foot-hair. Nature, 405:681-685, 2000.], possibly in combination with capillary forces [G. Huber, H. Mantz, R. Spolenak, K. Mecke, K. Jacobs, S. N. Gorb, and E. Arzt. Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements. Proceedings of the National Academy of Sciences, 102: 16293-16296, 2005.], [V. Sun, P. Neuzil, T. Kustandi, S. Oh, and V. D. Samper. The nature of the gecko lizard adhesive force. Biophysical Journal: Biophysical Letters, L14-L17, 2005.]. The complexity of the structure of these fibers differs among the species of animal. For large lizards such as the Tokay gecko the fibers take on a complicated branched structure, whereas for lighter animals such as spiders and anoles the structure is a simple array of angled high aspect ratio microfibers [E. Arzt, S. Gorb, and R. Spolenak. From micro to nano contacts in biological attachment devices. Proceedings of the National Academy of Sciences, 100(19):10603-06, September 2003.]. Some Gecko species have adhesion strength capabilities as high as 100 kPa [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full. Adhesive force of a single gecko foot-hair. Nature, 405:681-685, 2000.]. In Geckos, the oriented fibers are made of a stiff biomaterial (beta-keratin) with a Young's modulus of approximately 4 GPa [B. N. J. Persson. On the mechanism of adhesion in biological systems. Journal of Chemical Physics, 118:7614-7621, April 2003.] and have diameters from 0.2 to 5 μm [E. Arzt, S. Gorb, and R. Spolenak. From micro to nano contacts in biological attachment devices. Proceedings of the National Academy of Sciences, 100(19):10603-06, September 2003.]. The structure and material properties such as Young's modulus allow the fibers to individually bend and adapt to a wide variety of surface roughnesses and also to return to their original shape after release from the surface. Fabrication of similar synthetic structures would enable the production of long-lifetime reusable fibrillar adhesives with broad applications.
The enhanced adhesion from fibrillar surfaces has been studied and described in terms of fracture mechanics, elastic beam theory, and surface interaction forces [A. Crosby, M. Hageman, and A. Duncan, Controlling polymer adhesion with “pancakes”, Langmuir, 21:11738-11743, 2005.], [T. Tang, C.-Y. Hui, and N. J. Glassmaker, Can a fibrillar interface be stronger and tougher than a non-fibrillar one? Journal of The Royal Society, Interface, 2(5):505-516, 2005.], [C. Hui, N. J. Glassmaker, T. Tang, and A. Jagota. Design of biomimetic fibrillar interfaces: 2. mechanics of enhanced adhesion, Journal of The Royal Society, Interface, 1:35-48, 2004.], [B. N. J. Perrson and S. Gorb. The effect of surface roughness on the adhesion of elastic plates with application to biological systems. Journal of Chemical Physics, 119(21):11437-11444, 2003.], [B. N. J. Persson. On the mechanism of adhesion in biological systems, Journal of Chemical Physics, 118:7614-7621, April 2003.], [N. J. Glassmaker, A. Jagota, C.-Y. Hui, and J. Kim. Design of biomimetic fibrillar interfaces: 1. making contact. Journal of The Royal Society, Interface, 1(1):23-33, November 2004.], [J. Y. Chung and M. K. Chaudhury. Roles of discontinuities in bioinspired adhesive pads. Journal of The Royal Society Interface, 2:55-61, 2005.], including analysis of the effects of tip shape and fiber size [R. Spolenak, S. Gorb, H. Gao, and E. Arzt. Effects of contact shape on the scaling of biological attachments, Proceedings of the Royal Society A, 461:305-319, 2005.], [H. Gao and H. Yao. Shape insensitive optimal adhesion of nanoscale fibrillar structures. Proceedings of the National Academy of Sciences, 101(21):7851-7856, May 2004.]. Work has also been conducted to create synthetic fiber adhesives via various fabrication techniques. Since van der Waal's forces are universal, a wide variety of materials and techniques may be used to construct the fibers. Methods such as electron-beam lithography (A. K. Geim, S. V. Dubonos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval. Microfabricated adhesive mimicking gecko foot-hair. Nature Materials, 2:461-463, 1 Jun. 2003.], micro/nanomolding [N. J. Glassmaker, A. Jagota, C.-Y Hui, and J. Kim. Design of biomimetic fibrillar interfaces: 1. making contact. Journal of The Royal Society, Interface, 1(1):23-33, November 2004.], [M. Sitti and R. Fearing. Synthetic gecko foot-hair micro/nano-structures as dry adhesives, Journal of Adhesion Science and Technology, 17(58):1055-741073, May 2003.], [C. Majidi, R. Groff, and R. Fearing, Clumping and packing of hair arrays manufactured by nanocasting, Proc. of the ASME International Mechanical Engineering Congress and Exposition, pages 1-6, 2004.], [C. Menon, M. Murphy, and M. Sitti. Gecko inspired surface climbing robots, Proc. of the IEEE Int. Conf. on Robotics and Biomimetics, pages 431-436, August 2004.], and self-assembly are employed to fabricate fibers from polymers [K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full. Evidence for van der waals adhesion in gecko setae, Proceedings of the National Academy of Sciences, 99:12252-56, September 2002.], [M. Sitti and R. Fearing. Synthetic gecko foot-hair micro/nanostructures as dry adhesives. Journal of Adhesion Science and Technology, 17(5):1055-1073, May 2003.], polymer organorods [M. T. Northen and K. L. Turner, A batch fabricated biomimetic dry adhesive. Nanotechnology, 16: 1159-1166, 2005.], and multi-walled carbon nanotubes [Yo Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A. Majumdar. Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 24:331-335, 2006.], [B. Yurdumakan, N. R. Raravikar, P. M. Ajayanb, and A. Dhinojwala. Synthetic gecko foot-hairs from multiwalled carbon nanotubes. Chemical Communications, pages 3799-3801, 2005.].
U.S. Pat. No. 6,872,439 describes a variety of methods for the fabrication of microfibers, including the fabrication of angled microfibers. The methods include the fabrication of negative templates by using substrates with fabricated or self-organized high aspect ratio holes. These holes can be made by imprinting the desired shape using single sharp probes, or by using optical lithography, deep reactive ion etching (DRIE) with thermal oxidization processing, black silicon etching, laser micro/nanomachining, electron-beam lithography, nano-imprinting, or soft-lithography. A second approach is through the use of a positive template that is fabricated by molding already existing or fabricated high aspect ratio stiff micro/nano-structures that are not appropriate to use directly as synthetic hair. These micro/nano structures, for instance, could be carbon nanotubes, nanowires, or nanorods.
Several methods are disclosed to achieve oriented fibers. First, a soft surface, such as wax, may be indented by a sharp probe at an angle. Another method is to shear a molded template under stress and at a specific temperature to plastically deform it to a desired angle θ.
U.S. patent application Ser. No. 10/863,129 (published as US 2005-0271869 A1) and Ser. No. 10/982,324 (published as US 2005-0271870 A1) disclose a method for forming hierarchical structures of microfibers with smaller microfibrils attached to the end. In one embodiment, these applications describe a method to fabricate nanostructures that are angled. This method relies on the insertion of oriented fibers into a liquid polymer which is then cross-linked to provide a final microfiber embedded substrate. Smaller microfibrils are then microimprinted or attached to the top surface of this substrate. Fabrication of aligned microfibrils with controlled density and embedding them inside a polymer matrix are not described.
The microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Moreover, they cannot efficiently and controllably produce angled fibers. Adhesion and overall work of adhesion of the microfiber arrays are measured and compared with the models to observe the effect of fiber geometry and preload.
Accordingly, there is a need for improved dry adhesives and improved methods for making dry adhesives. In particular, there is a need for dry adhesives having greater adhesive forces and improved durability. In addition, there is a need for methods of making dry adhesives with lower costs of production. Those and other advantages of the present invention will be described in more detail hereinbelow.